This invention relates to a process of producing polytrimethylene terephthalate (PTT) with an intrinsic viscosity of at least 0.75 dl/g by esterification of terephthalic acid (TPA) with trimethylene glycol (TMG; this is also referred to as 1,3-propanediol, PDO) in the presence of a catalytic titanium compound to obtain an esterification product, precondensation of the esterification product to obtain a precondensation product, and polycondensation of the precondensation product to obtain PTT.
Processes of producing PTT are known (U.S. Pat. Nos. 2,456,319; 4,611,049; 5,340,909; 5,459,229; 5,599,900).
For instance, U.S. Pat. No. 4,611,049 describes the use of a protonic acid as co-catalyst for accelerating the polycondensation of TMG and dimethyl terephthalate, where the addition of p-toluene sulfonic acid in a concentration of 50 mmol % effects an increase of the maximum achievable intrinsic viscosity of 0.75 dl/g in a batch process catalyzed with 50 mmol % tetrabutyl titanate to 0.90 dl/g.
U.S. Pat. No. 5,340,909 proposes to achieve an improvement of the polycondensation capacity and the color of the polytrimethylene terephthalate by using a tin catalyst, which together with titanium can already be present in the esterification. Statements on the influence of recirculation of the vapor condensates obtained during the polycondensation on the polycondensation capacity of the reaction melt cannot be found in U.S. Pat. No. 5,340,909.
U.S. Pat. No. 5,459,229 proposes to reduce the concentration of acrolein in the vapors by adding alkalines to the condensates produced during the esterification of trimethylene glycol and terephthalic acid U.S. Pat. No. 5,459,229 does not contain any details concerning the esterification and polycondensation.
U.S. Pat. No. 5,599,900 describes a process of producing polytrimethylene terephthalate, where in the presence of an inert stripping gas either after the transesterification or after the esterification a polytrimethylene terephthalate with a degree of polymerization of 64 is synthesized. Moreover, it is desired to also adjust higher molecular weights but this is not proven by experiment.
WO 97/23543A describes a process of producing polytrimethylene terephthalate, where it is provided to produce a preproduct with an intrinsic viscosity of 0.16 dl/g by means of transesterification. This preproduct is converted to pastilles by means of dripping, which pastilles directly crystallize at crystallization temperatures up to 130xc2x0 C. The actual polymer is produced subsequently by solid phase condensation. It is disadvantageous that a high amount of trimethylene glycol and oligomers gets into the process gas and must be recovered or burnt, which is expensive.
U.S. Pat. No. 5,798,433 describes a process of producing PTT by direct esterification of terephthalic acid with 1,3-propanediol and subsequent precondensation and polycondensation. The PTT produced is obtained using a combination of titanium and antimony catalysts. The quantity of the required catalyst is very high and causes severe disadvantages in the product quality especially with regard to the product thermal stability.
From U.S. Pat. No. 4,011,202 the use of glycol jet pumps is known. However, the use of TMG jets is not detailed.
It can be seen that it would be advantageous to create a melt phase process of producing PTT with an intrinsic viscosity between 0.75 and 1.15 dl/g and a good thermal stability, and to achieve at the same time an efficiently long service life of the filters when the polymer melt is filtered prior to processing the same to form the end products. The process may be a batch or continuous process. Additionally, the PTT process should also allow the recycling of TMG and oligomer byproducts.
The characteristic features of this process, which comprises the catalytic esterification of TPA with TMG, precondensation of the esterification product and polycondensation of the precondensation product, are as follows:
1) The esterification is performed in at least 2 stages, one initial stage and at least one second, subsequent stage connected to a process column.
2) The catalyst used for esterification and polycondensation is a titanium compound, preferably in a stabilized liquid formulation, which is prepared from a catalytic titanium compound, an organic acid and TMG as solvent, in such way that the liquid catalyst feed contains less than 5 percent by weight (wt %) titanium
3) The catalyst used for esterification in the first, initial stage can be alternatively a Ti containing liquid reaction product from TPA and TMG with a degree of esterification of at least 97%, which may be recycled from a later reaction stage and fed to the initial esterification stage together with the raw materials.
4) A defined quantity of the described liquid catalyst feed is introduced into the first, initial esterification stage and separately a second defined quantity of the liquid catalyst feed is added to the at least one subsequent stage of esterification.
5) a major quantity between 65 and 100 wt % of said liquid catalyst feed containing 35 to 110 ppm titanium may be introduced into the at least one subsequent esterification stage, which is operated at a temperature of 245 to 260xc2x0 C. and a pressure of 0.7 to 1.5 bar,
6) a minor quantity of said liquid catalyst feed containing 0 to 40 ppm titanium and up to 35% of the total catalyst may be directly fed to the initial esterification stage, which direct catalyst feed can be partially or completely substituted by the same quantity of catalyst in a reaction product, which may be recycled from any further reaction stages and which is mixed with the raw materials for further reaction in said initial esterification,
7) In the first, initial esterification stage a total molar feed ratio of TMG/TPA of 1.15 to 2.5, an amount of titanium of 0 to 40 ppm, which is in maximum 35% by weight of the total amount of catalyst, a temperature of 240 to 275xc2x0 C. and an absolute pressure of 1 to 5.05 bar, preferably 3.5 bar, are adjusted, whereby the reaction is continued until 90 to 95% of the TPA is esterified.
8) In the at least one subsequent esterification stage an additional amount of titanium of 35 to 110 ppm, which is 65 to 100% by weight of the total amount of catalyst, a temperature of 245 to 260xc2x0 C. and an absolute pressure of 0.7 to 1.5 bar are adjusted, whereby the reaction is continued until 97 to 99% of the TPA is esterified.
9) The precondensation is performed at a temperature of 245 to 260xc2x0 C. under a reduced pressure in the range from 2 to 200 mbar.
10) The polycondensation is carried out in the melt phase at a temperature increasing from the entry to the exit of the polycondensation reactor from 250 to 270xc2x0 C. and at an absolute pressure of 0.2 to 2.5 mbar.
11) For generating the vacuum of the precondensation and polycondensation vapor-jet pumps are used, which are operated with TMG vapor, and the vapors sucked off and said TMG vapors are compressed by the vapor jet pumps and condensed by spraying them with a liquid which predominantly consists of TMG, for example the condensate from these spray condensers and optionally fresh make-up TMG.
The feed amount of titanium in the first, initial esterification stage preferably is in the range from 5 to 25 ppm. As the catalytic titanium compound to prepare the catalyst liquid there may preferably be used titanium tetrabutylate or titanium tetraisopropylate. As advantageous catalytic titanium compounds there may for instance also be used any catalytic titanium compound, such as titanium alkylates and their derivatives, like tetra-(2-ethylhexyl)-titanate, tetrastearyl titanate, diisopropoxy-bis(acetyl-acetonato)-titanium, di-n-butoxy-bis (triethanolaminato)-titanium, tributyl monoacetyltitanate, triisopropyl monoacetyltitanate or tetrabenzoic acid titanate, titanium complex salts, like alkali titanium oxalates and malonates, potassium hexafluorotitanate, or titanium complexes with hydroxycarboxylic acids such as tartaric acid, citric acid or lactic acid. Also special catalysts as titanium dioxidexe2x80x94silicon dioxidexe2x80x94co-precipitate or hydrated alkaline containing titanium dioxide can be used. Equivalent zirconium catalysts could also be used.
The solvent which is used in the liquid catalyst feed is TMG, in which for stabilization reasons a C4 to C12 dicarboxylic acid is dissolved in quantities below its saturation concentration at ambient temperature.
The organic di-acid which is preferably used for the liquid catalyst feed is selected from terephthalic acid, isophthalic acid or another C4-C12 aromatic or aliphatic dicarboxylic acid. Preferably the C4 to C12, dicarboxylic acid is incorporated in the PTT and does not act as chain stopper. A further embodiment consists of solutions of catalyst in TMG in which a C2 to C12, preferably C2 to C8, monocarboxylic acid is dissolved below its saturation concentration. The preferred monocarboxylic acid is acetic acid.
As further embodiment of the invention the catalyst liquid can be a Ti containing liquid reaction product from TPA and TMG with a degree of esterificaiion of at least 97%. This product is recycled from a later reactor stage and mixed to the first, initial esterification process together with the raw materials. In the continuous process the recycled product amounts to 5 to 40 wt %, more preferably to 10 to 30 wt % of the nominal throughput. In the case of the batch process the amount of recycled product lies between 25 and 85 wt %, preferably between 35 and 70 wt % of the nominal batch size. This option of the invention includes reaction products which may be withdrawn at any point between the exit from the second stage of esterification and the entry into polycondensation, and which are used as liquid catalyst feed for the first initial esterification stage.
The second portion of the catalyst may be fed after the esterification step. An important aspect of the invention consists in that in the initial stage of esterification a specific combination of parameters is used.
The described special catalyst liquid is well proven at temperatures within the range of 250 to 270xc2x0 C., an elevated molar feed ratio of TMG to TPA between 1.15 and 2.5, preferably between 1.5 and 2.4, and a pressure of 1 to 5.0 bar, preferably 3.5 bar. Under these conditions only a minor formation of non-filterable particles occurs independently whether delustering agents, like TiO2, or other additives are used. This is particularly necessary in the production of fibers.
In accordance with a further preferred aspect of the invention, the first initial stage of esterification is conducted to a TPA conversion of 90 to 95%, and the second stage of esterification raises the TPA conversion to 97 up to 99%. Late in the second stage of esterification it has been assured that the last particles of solid TPA from the paste are completely dissolved and the melt is clear and bright.
The catalyst liquid introduced into the second or further stages of esterification is preferably a clear solution. These above mentioned conditions enable low filter values of the PTT.
The process can be a continuous or a batch process. In the discontinuous process the initial process cycle with a transiently heterogeneous reaction mixture and a limited TPA conversion of below 95%. is considered as the first, initial stage of esterification, while the later reaction cycle in a homogeneous melt phase at a TPA conversion of at least 97% represents the at least one subsequent esterification stage. Accordingly the second part of the liquid catalyst feed is added when the TPA has been esterified to at least 95%, preferably to more than 97%.
The precondensation, especially in the continuous process, is preferably divided into two pressure sections to provide an optimum condensation process. The first stage of precondensation is performed between 50 and 200 mbar, the second stage between 2 and 10 mbar.
It is particularly advantageous when the polycondensation of the prepolymer melt is performed at a pressure of 0.3 to 0.8 mbar.
Preferably the polycondensation reactor is a disc ring reactor or a cage type reactor, which allows the formation of steadily renewed, large film surfaces of the reaction product and facilitates by this the evaporation of the volatile products. Under these conditions, increased intrinsic viscosities in the range from 0.75 to 1.15 dl/g are possible.
In keeping with the desire to maintain control of the temperature to which the oligomer or polymer is exposed during each stage of the process including polycondensation, the temperatures of the walls of the reaction vessels are controlled, as contact of the polymer with excessively hot vessel walls is a potential cause of polymer degradation. It is preferred that a heat transfer medium (HMT) be used to control the temperature of the reactor walls and that the HMT temperature be not more than 300xc2x0 C., preferably not more than 290xc2x0 C.
It is surprisingly found out that in accordance with the inventive process very advantageous filter values of 0 to 40 bar-cm2/kg can be realized (for the determination of filter value, see below).
In accordance with a further object of the invention it is provided that the condensates of the spray condensers, optionally after the distillation of low boilers, are recirculated into the first initial and possibly further stages of esterification. In this way, a substantial reduction of the losses in raw materials is achieved.
In accordance with a further preferred embodiment of the invention it is provided that the PTT contains up to 20 wt % comonomer units derived from other dicarboxylic acids and/or diols. As other dicarboxylic acid there may for instance be used adipic acid, isophthalic acid or naphthalene dicarboxylic acid. As diols there may for instance be used ethylene glycol, diethylene glycol, triethylene glycol, butylene glycol, polyglycols, as well as cyclohexane dimethanol. In this way, the end product can be adapted to the respective application relatively easily.
A further embodiment of the invention consists in that at any point before the end of the polycondensation in the melt phase usual additives such as delustering agents and/or color agents and/or branching agents and/or stabilizers can be added. By means of this measure, the number of the applications of the end product will be increased in connection with a particular viscosity adjustment.
In accordance with the invention, a polyester-soluble cobalt compound, for instance cobalt acetate, and/or polyester soluble organic dyes can be used as color agent or blue toner. As stabilizer a phosphorus compound is added with up to 20 ppm phosphorus, based on PTT, in connection with the cobalt compound and up to 10 ppm phosphorus without any addition of cobalt. By this amounts of phosphorus the catalysis of the thermal degradation of the PTT melt by ions of heavy metals including of the cobalt is stopped because of the formation of neutral phosphorus salts. In special cases the addition of phosphorus may be omitted completely; this depends on the quality of the raw materials, the construction materials of the equipment as well as on the final product application.
Other stabilizers include hindered phenolic esters such as those selected from the group consisting of methyl(3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, octadecyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, tetrakis(methylene -3,5-di-t-butyl-4-hydroxyphenyl) propionate))methane, 1,6-hexamethylene bis(3-(3,5-di-t-butyl-4-hydroxyphenyl) ropionate), triethyleneglycol bis(3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionate) and 1,3,5-tris(3,5-di-t-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione.
Also, there may be used as stabilizers aromatic organophosphites including those selected from the group consisting of tris(2,4-di-t-butylphenyl)phosphite, bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite and 2-[[2,4,8,10-tetrakis(1,1-dimethylethyl)dibenzo [d,f][1,3,2]-dioxaphosphepin-6-yl]oxy]-N,N-bis[2-[[2,4,8,10-tetrakis(1,1-dimethylethyl)dibenzo[d,f][1,3,2]dioxaphosphepin-6-yl]oxy]-ethyl]ethanamine.
A further optional aspect of the invention is that carboxylic acids with three or more COOH groups, polyfunctional acid anhydrides, or polyfunctional alcohols with three or more OH groups, or carboxyphosphonic acids or the esters thereof in concentrations below 5000 ppm can be incorporated into the polymer as branching agents. For polycarboxylic acids and polyalcohols concentrations below 1000 ppm are sufficient in most cases. These compounds can particularly be used for adjusting or raising the intrinsic viscosity.
The PTT can be directly processed to fibers, films or other molded materials. In accordance with a further embodiment of the invention is provided that after the polycondensation in the melt phase the PTT is granulated and crystallized. The resulting granulate can also be thermally treated in the solid state for further IV build up or for devolatilizing low molecular organic products such as acrolein, allyl alcohol, and water. The PTT granulate can then be processed to fibers, filaments, films or molded articles.
The processed products, i.e. fibers, filaments, films, molded articles or chips, are characterized by an IV of 0.8 to 1.1 dl/g, a filterability of  less than 40 bar-cm2/kg and a thermal stability (as defined below) of  greater than 80